Chemistry and catalysis advances in organometallic chemistry and catalysis
[33] fragments (Scheme 14.18). Among them, the most successful planar chiral ferrocenyloxazolinylphosphines 19
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29 [33] fragments (Scheme 14.18). Among them, the most successful planar chiral ferrocenyloxazolinylphosphines 19 (Fc- PHOX) are versatile ligands that have been applied to a wide range of asymmetric catalytic reactions, such as hydrogenation [34], transfer hydrogenation [35], and α-alkylation of ketones [36], the oxidative kinetic resolution of racemic alcohols [37], allylic substitutions [38], Heck reactions [39], and 1,3-dipolar cycloadditions [40]. One should note that systems 19–29 (except 23) not only have a stereogenic center in the oxazoline ring, but also possess planar chirality, and these readily form with metals rigid six-membered chelates, which is believed to favor further N O R' Fe N O R' PR 2 PAr
2 18 19 O N R' N PR 2 N O N R' PAr 2 N S N R PAr 2 N O N R 1 PAr 2 R 2 R 2 R 2 O N R 1 R 2 P O N R' R 2 P N N O R PPh
2 N O R' N PR 2 N O R N PPh 2 27 23 20 21 22 26 25 24 29 28 O N R' S PR 2 Scheme 14.18 192 COORDINATION CHEMISTRY OF OXAZOLINE/THIAZOLINE-BASED P,N LIGANDS E N
1 O R 2 P R 2 O N R 2 O PR 2 R 1 R 1 R 3 O N R' PR 2 O N R' O P R 1 R 1 R 2 O O O O = O O R 1 R 2 R 2 R 1 O O O N R' O PR 2 O N R 1 O P R 2 N N N N = N N Ph 30 31 32 33 34 35 Scheme 14.19 chirality transfer. Ligands of types 20–24 have recently led to highly enantioselective iridium catalysts for the hydrogenation of α,β-unsaturated esters [26]. The P-donor can also be introduced at the 4-position of the oxazoline/thiazoline ring, which effectively enhances the scope of such chiral systems, and the corresponding ligand subunits D and F are shown in Scheme 14.19. Usually, ligands containing subunit D possess only chiral center(s) in the N-containing moiety, as in 30 [41], 31 [11], 32 [42], and 33 [43] (Scheme 14.19), and such ligands exhibit high selectivity in asymmetric catalysis. The results obtained with rhodium(I) complexes of ligand 31 in the asymmetric hydrogenation of functionalized olefins using propylene carbonate as a solvent were similar to or better than those obtained in standard solvents, illustrating its potential as a solvent for asymmetric hydrogenation reactions. The asymmetric hydrogenation of nonfunctionalized olefins appears particularly interesting, as nonpolar products are formed, which can be removed by extraction, and the catalyst can be readily recycled [11]. In 34 and 35, there is one more element of chirality in the phosphite or phosphinite moiety, such as the binaphthyl system [44]. These modifications could potentially influence the origin of the stereochemistry in the asymmetric catalysis process, and sometimes excellent results have been obtained (quantitative conversions and enantiomeric excess values higher than 99%) in the case of 34 (R 1 = Ph, R
2 = H, R = Ph) that is better than other known catalyst systems for the asymmetric hydrogenation of unfunctionalized 1,1-disubstituted terminal alkenes [44]. In view of the improvements generally observed in homogeneous catalysis when going from subunit C to E (Scheme 14.14), it was felt interesting to introduce similar structural modifications from subunit D to F, which feature a cyclic backbone. Ligands E and F chelate metal centers to form a six-membered ring, with the imine double bond included in the chelate ring in E, in contrast to F [45]. Corresponding structural studies on such systems are still rare. Ligands containing a subunit F are shown in Scheme 14.20, and most of them exhibit high enantioselectivity in diverse asymmetric syntheses [46].
Research on ligand design based on modifications of the spacer group between the P-donor and the oxazoline/thiazoline was extended further, generally to investigate the steric influence of the spacer on stereochemistry. Several ligands that can form seven-, eight-, or nine-membered metal chelates have been prepared and the resulting complexes sometimes display high catalytic activity. Selected ligands such as 43, 44, 45, and 46 are shown in Scheme 14.21, in which 43 was successfully applied in the hydrogenation of α,β-unsaturated ketones [47] and Heck reactions [48], 44 in asymmetric Mannich reactions [49], 45 in hydrogenation of exocyclic α,β-unsaturated carbonyl compounds [50], and 46 in the hydrogenation of imines
P,N-CHELATING LIGANDS BASED ON OXAZOLINE/THIAZOLINE SYSTEM 193 S N Ph PAr
2 O O O Ph R 2 P N O R' S N Ph O PAr 2 O O O Ph P N O R' O O O O = O O R 1 R 2 R 2 R 1 O N R' PR 2 O N Ph O PAr 2 S N Ph PAr
2 O O R R
37 38 39 40 41 42 Scheme 14.20 N O PR 2 R R 1 R 1 43 44 N O R' PR 2 R 2 P N O R N O R' PR 2
46 Scheme 14.21 [23] and of α,β-unsaturated carboxylic acids [51]. It is noteworthy that the Ir(I) complexes of ligand 46 (R = 3,5-tBu 2 C 6 H 3 , R = Bn, iPr, Me, H) exhibit very high activity and selectivity in asymmetric hydrogenation of (R)-methylcinnamic acid (quantitative conversion, almost quantitative yields, and enantiomeric excess (ee) values higher than 99%). This was the first application of such kind of ligands (spiro phosphino-oxazoline) to the highly enantioselective Ir-catalyzed hydrogenation of α,β-unsaturated carboxylic acids. Although the oxygen or sulfur atom of the heterocycle part of the P,N ligand was never observed to participate in direct coordination to the metal, it may influence the donor properties of the nitrogen of the oxazoline or thiazoline rings. Only few examples in which the oxygen/sulfur atom of an oxazoline/thiazoline ring coordinated to a metal center have been structurally characterized [52], and a recent example is shown in Scheme 14.22 [52]. N N N N S N S Pd Scheme 14.22 194 COORDINATION CHEMISTRY OF OXAZOLINE/THIAZOLINE-BASED P,N LIGANDS N O
2 P PPh 2 N O Ph 2 P PPh 2
48 Scheme 14.23 14.2.4 Dppm-Type Ligands Containing an Oxazoline Moiety In view of the rich chemistry of the dppm [bis(diphenylphosphino)methane], and related short-bite ligands [53], the introduction of additional functionalities at the PCP carbon is anticipated to bring about further structurally diversity and possibilities in coordination chemistry. Among the C-substituted dppm-type ligands, only few examples in which the substituents are N-containing heterocycles, such as imidazole [54], quinoline [55], terpyridine [56], indole [57], and, in particular, 2-pyridine have been reported [58]. In order to extend this chemistry, we have chosen oxazoline and its derivatives as substituents at the PCP carbon. The diphosphine ligands 47 [4, 59] and 48 [4] were found to behave not only as C-substituted dppm but also as P,N ligands and their coordination behavior has only recently begun to be investigated (Scheme 14.23). The P,P-chelating mode, which is a typical coordination mode for dppm, was also found for anionic ligand 47 in the P,P bis-chelated Pt(II) [4] and Pd(II) [59] complexes. In view of the presence of free nitrogen donors, further reaction with two equivalents of [AuCl(tht)] led to a trinuclear, bimetallic complex in which both anionic ligands 47 displayed a P,N-chelating/P,P bridging mode as a result of ligand rearrangement (Scheme 14.24). We also characterized a platinum complex in which the anionic ligands 47 exhibit a P,N-chelating and a P,P-chelating mode, the metal center being involved in a five-membered and a four-membered chelating ring, respectively (Scheme 14.25). As a result of an unexpected metal-induced phosphoryl migration reaction from carbon to nitrogen, ligand 47 can display different isomeric forms that lead to the formation of a cationic bis-chelated platinum complex containing a rearranged and an intact ligand 47 as part of a six-membered and a four-membered chelate (Scheme 14.26). The versatility in bonding mode of functional dppm-type ligands bearing an oxazoline substituent on the PCP carbon atom was thus clearly demonstrated in Pd(II) and Pt(II) complexes with the neutral or the monoanionic forms of the P,N ligand. The chelating, gem-diphosphine form resulted from migration of one of the PPh 2 groups from a phosphino-oxazoline nitrogen atom to carbon, whereas metal coordination triggers the reverse migration. The tautomeric/isomeric forms of ligand 47 are shown in Scheme 14.27 [59]. M N O Ph 2 P Ph 2 P Au Cl N O PPh 2 Ph 2 P Au Cl 2 AuCl O N Ph 2 P P Ph 2 M N O Ph 2 P P Ph 2 M = Pt, Pd M = Pd
Scheme 14.24 O N Ph 2 P P Ph 2 Pt N Ph 2 P O Ph 2 P Scheme 14.25 REFERENCES 195 O H N Ph 2 P P Ph 2 Pt P Ph 2 N Ph 2 P O 2+
N O
2 PPh
2 N O Ph 2 P PPh 2 N O PPh
2 PPh
2 N O Ph 2 P PPh 2 H H Tautomeric / Isomeric Forms CH 2
2 Stable as free ligands Stabilization by metal coordination required Scheme 14.27 (See insert for color representation of the figure.) SUMMARY The chemistry of P,N-chelating ligands based on oxazoline/thiazoline heterocycles has recently experienced major developments. In this chapter, we wished to provide a selection of recent examples illustrating the bonding and structural diversity encountered as a function of the size of the chelate ring they form on coordination. These features are expected to significantly influence, for example, the catalytic and photophysical properties of their metal complexes. While some of these systems have been known for almost 20 years, research continues to be active, especially toward catalytic applications [60]. Furthermore, oxazoline-substituted dppm ligands such as 47 have been studied and revealed unexpected coordination behavior (Scheme 14.24–14.26). The P,P bis-chelated platinum and palladium complexes shown in Scheme 14.24 are luminescent, probably owing to the electronic delocalization over the anionic ligand. This delocalization can be tuned by linking different functionalities to the PCP carbon, which should allow promising developments toward new C-substituted dppm ligands.
This work was supported by the CNRS, the Minist`ere de l’Enseignement Sup´erieur et de la Recherche, the Institut Franc¸ais du P´etrole, Energies Nouvelles (IFPEN), and the International Centre for Frontier Research in Chemistry, Strasbourg (icFRC, www.icfrc.fr). REFERENCES 1. Braunstein, P. J. Organomet. Chem. 2004, 689 , 3953. 2. For a review, see: Braunstein, P.; Naud, F. Angew. Chem. Int. Ed. 2001, 40 , 680. 3. For example, see: (a) Maggini, S. Coord. Chem. Rev. 2009, 253 , 1793; (b) Mata, Y.; Pamies, O.; Dieguez, M. Adv. Syn. Catal. 2009, 351 , 3217; (c) Willms, H.; Frank, W.; Ganter, C. Organometallics 2009, 28 , 3049; (d) Benito-Garagorri, D.; Kirchner, K. Acc. 196 COORDINATION CHEMISTRY OF OXAZOLINE/THIAZOLINE-BASED P,N LIGANDS Chem. Res. 2008, 41 , 201; (e) Roseblade, S. J.; Pfaltz, A. Acc. Chem. Res. 2007, 40 , 1402; (f) Braunstein, P.; Chem. Rev. 2006, 106 , 134; (g) Guiry, P. J.; Saunders, C. P. Adv. Syn. Catal. 2004, 346 , 497; (h) Chelucci, G.; Orru, G.; Pinna, G. A. Tetrahedron 2003, 59 , 9471; (i) Pfaltz, A.; Blankenstein, J.; Hilgraf, R.; Hormann, E.; McIntyre, S.; Menges, F.; Schonleber, M.; Smidt, S. P.; Wustenberg, B.; Zimmermann, N. Adv. Syn. Catal. 2003, 345 , 33; (j) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33 , 336; (k) Slone, C. S.; Weinberger, D. A.; Mirkin, C. A. Prog. Inorg. Chem. 1999, 48 , 233; (l) Espinet, P.; Soulantica, K. Coord. Chem. Rev.
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